Option 1: Atmospheric disturbance refers to changes in weather patterns and atmospheric conditions. While it can cause short-term variations in sea level, it is not a significant contributor to long-term sea-level change.

Option 2: Change in marine water density can affect regional variations in sea level, but it does not directly contribute to long-term sea-level change.

Option 3: Melting of icebergs can contribute to short-term fluctuations in sea level, particularly at local scales. However, it is not a primary cause of long-term sea-level change.

Option 4: The melting of ice sheets, such as those in Greenland and Antarctica, is a significant cause of long-term sea-level change. As these massive ice sheets melt, they add excess water to the oceans, resulting in a rise in sea level over time. This process is currently a major concern due to climate change and global warming.

In conclusion, option 4 - Melting of ice sheets is the correct answer as it is the primary cause of long-term sea-level change.